Correct valve timing plays a considerable part in the output and efficiency of an engine, for if the inlet valve, for example, opens too late, the cylinder will not receive a full charge. If it opens too early the hot gases in the cylinder will ignite the gas in the carburetor and cause back-firing. Should the exhaust open too late, the retention of the hot gas in the cylinder is likely to cause overheating. The timing of the valves is usually expressed in degrees of the circle described by the crank-pin, or the angle formed by the crank with the center line of the cylinder at the time the valve is to open or close. (137) Valve Setting on Stationary Engines. The exhaust should open when the crank lacks 30° of completing the outer end of the power stroke, that is, the crank should make an angle of 30° with the center line of the cylinder when the exhaust valve begins to open, and should be inclined =AWAY= from the cylinder. Some makers have the exhaust open a little later in the stroke, but little is to be gained with a later opening as the retention of the charge beyond 30° heats the cylinder and does very little towards developing power. The only advantage of the late opening is that the valve opens against a lower pressure and causes slightly less wear on the parts. The exhaust valve should close 5° =AFTER= the crank has passed the =INNER= dead center on the exhaust or scavenging stroke, although some makers close the valve exactly on the dead center. The 5° should be given to allow the gas all possible chance of escape. The piston is said to be on the inner dead center when it is in the cylinder as far as it will go, and on the outer dead center when it is on the center nearest the crankshaft. The =INTAKE= valve should open about 5° =AFTER= the exhaust valve closes, or 10° after the crank passes the inner dead center. The inlet valve should =NEVER= open before the exhaust valve closes on a low speed engine. The above timing is for engines running 150–600 R.P.M. The automatic type of inlet valve, of course, cannot be timed, but attention should be paid to the strength and tension of the spring and the condition of the valve stem guides. The inlet valve should close 10° =AFTER= the crank passes the outer dead center in order that the cylinder be filled to the fullest possible extent. If the valve closed exactly on the dead center a partial vacuum will exist and the charge retained in the cylinder will be comparatively small, but if the valve remains open past this point the air would have time to completely fill the cylinder and develop the capacity of the engine. The longer the inlet pipe, the longer the inlet valve opening. (138) High Speed Engine Valve Timing.